1. Field of Invention
The present invention generally relates to surgical device design and fabrication and, more particularly, to hollow medical wires used as guidewires, catheters, and the like, and methods of constructing same.
2. Description of the Related Art
In the medical community, the continuing trend of less-invasive and noninvasive surgical techniques is driving the medical industry to explore new materials and processes for fabricating surgical instruments and devices having smaller size and better material properties. Examples of such instruments include angioplasty catheters incorporating balloons to dilate an occluded blood vessel. Other catheters are used to deploy stents or other types of therapeutic devices.
Because of the success and acceptance of procedures which utilize such catheters, new procedures are being developed which require variations and adaptations of previous catheter technology. For example, in the U.S., one of the more common applications for medical catheter technology is the xe2x80x9cover-the-wirexe2x80x9d balloon angioplasty catheter. In this application, the catheter is comprised of an elongate hollow body which has mounted on its distal end an inflatable therapy balloon. The catheter body in this case is typically constructed from a plastic material and is hollow (e.g., sometimes referred to as xe2x80x9cmicrotubingxe2x80x9d), both to supply inflation fluids to the balloon and to allow the catheter device to ride over a thin wire to the site to be treated. Thus, this medical device is referred to as an xe2x80x9cover-the-wirexe2x80x9d therapy catheter.
The thin wire over which the catheter rides is commonly referred to as a xe2x80x9cguidewire,xe2x80x9d obviously, because it guides the therapy balloon to the treatment location. Such medical guidewires are typically made from a solid construction, i.e., they are not normally hollow since they do not need to carry fluids to the therapy site. Such medical wires can be adapted to guide other types of therapy devices well, such as stents, atherectomy devices, laser devices, ultrasound devices, drug delivery devices, and the like.
Another type of balloon angioplasty device is referred to as a xe2x80x9csingle operatorxe2x80x9d balloon catheter. More common in Europe, this type of device rides along a guidewire with only a short section of the device (i.e., the xe2x80x9csingle operatorxe2x80x9d) actually riding completely over with the guidewire.
Another type of therapy balloon device which does not require a guidewire is referred to as a xe2x80x9cballoon-on-a-wirexe2x80x9d or a xe2x80x9cfixed-wire balloonxe2x80x9d catheter. The body of the catheter in this case is typically a hollow metallic wire (e.g., a xe2x80x9chypotubexe2x80x9d) or plastic wire, providing inflation fluid to the balloon mounted on the distal end. This type of therapy balloon device is less common in the U.S., being used in about less than 5% of the angioplasty procedures which are performed, compared with both over-the-wire and single operator type therapy balloon catheters which are used about 70% and 25% of the time, respectively.
In order to successfully perform the desired therapy using present catheter technology, there are a number of functional requirements which guidewires must exhibit. These are, not in any particular order of importance, as follows: pushability; trackability; torqueability; flexibility; and handleability. To the extent that a medical guidewire (or a guiding catheter or another similar guiding devices) exhibits one or more of these functional characteristics, it is more likely to be successful, both medically and commercially.
Pushability refers to the ability of a medical guidewire to be efficiently and easily pushed through the vasculature of the patient without damage thereto, but also without getting hung up, blocked, kinked, etc. Excessive force should not be necessary. The relative stiffness or rigidity of the material from which the wire is made is a key mechanical feature of the wire, at least with respect to its pushability. That is, the wire must be stiff enough to be successfully and efficiently pushed through the vessels to the treatment site, but not too stiff to cause damage. Likewise, a guidewire that is not sufficiently stiff or rigid will suffer xe2x80x9cprolapse.xe2x80x9d This condition occurs when the wire bends over on itself or strays down a branching vessel without progressing to its intended site. Thus, a wire that is too limp lacks sufficient strength to have good pushability characteristics, which are important in virtually all guidewire applications.
Trackability, in the case of guidewires, refers to the ability of the wire to have another device, such as a therapy catheter, efficiently pushed over it to a particular location. Thus, this is also an important feature of catheters which must also xe2x80x9ctrackxe2x80x9d efficiently over a guidewire. Time is usually of the essence with respect to many noninvasive therapy procedures since the blood flow of the patient may be interrupted partially or wholly, during such therapy. In addition, there are often a number of xe2x80x9cexchangesxe2x80x9d during such procedures in which one over the wire device is removed and replaced with anotherxe2x80x94both riding on the same guidewire. Thus, the ability of the guidewire to provide good tracking characteristics is important to the success of the wire. Again, the stiffness of the wire plays an important role in its trackability characteristics. Also, the lubricity of the material from which the wire is made will enhance its trackability by reducing frictional forces.
Torqueability refers to the ability of a medical wire to be accurately turned or rotated. It is often important, in traversing bends or turns, that the wire be rotated into a certain position. Ideally, a guidewire should exhibit 1:1 torqueability characteristics; for example, a one-quarter turn by the physician at the proximal end should result in precisely a one-quarter turn in the wire at the distal end. As one may expect, such ideal torqueability is very difficult to achieve in present medical wire technology.
Flexibility is another important characteristic of medical wires. It relates to the ability of the wire to follow a tortuous path, i.e., winding and bending its way through the tight turns of a patient""s vasculature. Small radius turns are found especially in the coronary arteries. Furthermore, diseased blood vessels become even more tortuous. For example, if plastic deformation in the wire results from traversing smaller, tight radius turns, the rigidity of the wire will be reduced. In addition, due to the permanent deformation, the straightness of the material is lost. It is therefore more likely to kink or possibly even break. Moreover, if the distal tip is bent, upon rotation, an injurious effect known as xe2x80x9cwhippingxe2x80x9d occurs as the distal tip of the wire beats against the inner wall of the vessel. Thus, the ability of a guidewire to traverse such tortuous paths without kinking, deformation, or damage to the vessel walls, is very important.
The handleability of medical wire relates to its feel during use. Especially important are reduced functional characteristics, such that the physician can actually xe2x80x9cfeelxe2x80x9d the tip as it is manipulated (including both torquing and pushing) from the proximal end. The therapeutic procedures using such wires require precise accuracy; thus, the movements of the wire must be smooth, controllable, and consistent. This is especially difficult to achieve in consideration of the long lengths of the wires (approximately 100 cm or more), and the fact that large sections remain outside the body while other sections are in the body and more or less hidden from view. Thus, it is important for the wire to be readily handled by the physician without kinking or requiring excessive forces or awkward movements.
It will also be noted that present guidewire technology also faces the challenge of extremely small dimensions. For example, guidewires used in therapeutic procedures performed in peripheral vessels often have an outer diameter of about 0.035 inches to around 0.038 inches. Wires used in connection with the coronary arteries are even smaller, ranging from 0.014 inches to 0.018 inches OD. Some devices even utilize guidewires with outer diameters of 0.009 inches. With these extremely small dimensions, it is very difficult to maintain the functional requirements for medical guidewires as outlined above.
Moreover, medical guidewires should also meet a number of structural requirements. The straightness of the wire is very important. If it is not as straight as possible, many functional features are lost, including most significantly the risk of damage to the vessel. Moreover, the roundness of the wire contributes to its accurate torqueability. Consistent wall thickness, lubricity, and many other structural and dimensional characteristics also play an important role.
In order to achieve these functional and structural characteristics, various materials have been proposed for the construction of the medical guidewires of the prior art. For the most part, elastic materials such as stainless steel have heretofore been used. Other so-called xe2x80x9csuperelastic materialsxe2x80x9d have also been utilized. Elasticity in a material is its ability to recover strain after deformation. High elasticity (or xe2x80x9csuper elasticityxe2x80x9d) therefore refers to the ability of the material to undergo deformation and to return to its original configuration without being permanently or xe2x80x9cplasticallyxe2x80x9d deformed. When such permanent or plastic deformation occurs, the structural integrity of the material is diminished (e.g., it loses, to some degree, its rigidity, and/or torqueability), and it assumes a new configuration (sometimes referred to as the xe2x80x9cpermanent setxe2x80x9d) from which subsequent loading begins. Moreover, the plastic deformation of a superelastic material may be accelerated through a number of cyclical deformations, sometimes referred to as fatigue. Such cyclical deformations can occur if the wire experiences a number of tight turns, such as is possible in the coronary arteries. Such superelastic materials include a variety of nickel titanium (NiTi) alloys, commonly referred to as xe2x80x9cnitinol,xe2x80x9d and other alloys exhibiting similar properties such as Cuxe2x80x94Znxe2x80x94Mn and Fexe2x80x94Mnxe2x80x94Si ternary alloys.
In medical guidewire applications, probably the most common of elastic materials is stainless steel. It provides good stiffness characteristics to supply desired pushability and torqueability. However, superelastic materials, including nitinol have also been suggested for medical wire applications. Although such elastic and superelastic materials provide acceptable results for typical applications, there is a need for more versatile and functional guidewires, especially as new therapeutic procedures are developed. In particular, there is a need for hollow medical guidewires which provide a lumen for inflation fluids, drug-delivery, device deployment and the like. As compared to the standard solid construction, such a hollow guidewire would provide much greater functionality or performance.
However, the challenges facing catheter designers today are greatly magnified in the case of a hypotube (even those made from a superelastic material) used to construct hollow guidewires. Furthermore, the adverse conditions experienced in actual practice may have a deleterious effect on the functional characteristics of the hollow wires, particularly those having extremely small diameters and thin wall thicknesses. For medical wire applications, such adverse conditions would include primarily the need to cyclically traverse a number of highly tortuous turns. This bending and twisting may result in plastic deformation which tests the true superelasticity of the material from which the wires are constructed. As a result, patients may suffer certain injuries, the full effects of which may not be known for years.
The aforementioned needs are satisfied by the medical wire device of the present invention which provides a highly versatile, efficient apparatus for performing angioplasty and other therapeutic procedures. In one embodiment, the present invention comprises a catheter having an elongate hollow body and a distally mounted occlusion device, preferably an occlusion balloon. The catheter body, which serves as a guidewire, comprises a hypotube constructed from a specially selected superelastic nitinol material. The nitinol material exhibits unique non-linear characteristics which provide unexpectedly high guidewire performance features. Moreover, because it is a hollow guidewire, the present catheter can deliver deployment media to the distal occlusion device, or assist in many other functions such as irrigation, drug delivery, and the like.
Thus, it will understood that the terms xe2x80x9ccatheterxe2x80x9d and xe2x80x9cguidewire,xe2x80x9d as used herein with reference to the medical device of the present invention are not to be limiting in any respect to their construction, materials, or functions, since the principals of the present invention are applicable to a wide variety of medical devices. The distal end of the catheters also provide it with a soft tip in order to avoid injury to the patient. Moreover, the body of the catheter, just proximal the occlusion balloon, is provided with a series of spaced radial radiopaque markers in order to provide visible reference points for the physician within the working space.
In another embodiment, the present invention comprises a catheter of similar construction in which the superelastic nitinol body does not necessarily serve as a guidewire. In yet another embodiment, the present invention comprises a composite medical wire device in which the wire is only partially constructed from the preferred non-linear superelastic nitinol material. In this embodiment, in order to achieve certain advantageous performance characteristics, the material may be joined with other materials (such as stainless steel, polymers or plastics, etc.) so as to be utilized for a given application. In a preferred embodiment, the distal section is constructed from the special nitinol material in order to achieve superior performance in softness and elasticity, but other sections may be formed from this material as well. In addition to such composite devices, the elongate body of the catheter can be internally constructed from the preferred nitinol material and then covered with a bilayer of stainless steel to form a concentric construction. Likewise, special heat treatments can be applied to the distal section to provide it with superior softness and flexibility. Also, such flexibility can be achieved through tapering to very small wall thicknesses. Thus, it will be understood that the principals of the present invention can be applied to medical wires of all types, partially or wholly metallic, hollow or nonhollow, etc., which may be used alone or in combination with other devices including therapeutic devices.
The preferred superelastic medical wire comprises a Nixe2x80x94Ti (nitinol) binary alloy having a nickel content between 50.0% and 51.5% by atomic weight, and preferably about 50.8%. The wire material can also be selected from a group of nitinol family ternary alloys comprising of Nixe2x80x94Tixe2x80x94V, Nixe2x80x94Tixe2x80x94Co, Nixe2x80x94Tixe2x80x94Cu, Nixe2x80x94Tixe2x80x94Cr, Nixe2x80x94Tixe2x80x94Nb, Nixe2x80x94Tixe2x80x94Pd or from a group of non-nitinol ternary alloys comprising Fexe2x80x94Mnxe2x80x94Si. A catheter or guidewire of the present invention constructed from the selected nitinol material exhibits outstanding performance characteristics. However, in addition, due to the special character of this material, the present invention wire devices also exhibit important characteristics of high recoverable strain and low modulus. Thus, recoverable strains in the range of about 1% to about 8% are feasible. This allows the present medical wire device to undergo high deformation without plastically deforming, a characteristic which is especially important in the case of thin-walled hollow hypotubes.
At the same time, due to the low modulus characteristics of the material, low stresses are induced as the device traverses the tortuous paths of the vasculature of the patient. Because of the low stress forces, reduced frictional forces are experienced; thus, the medical wire device of the present invention provides excellent handleability and xe2x80x9cfeelxe2x80x9d for the physician. In addition, there is reduced risk of injury. Moreover, in one embodiment, the nitinol material undergoes special heat treatment in order to achieve transformational effects. In this case, substantially constant stresses are maintained over a wide range of recoverable strains, improving even further the performance of the device.
Thus, in one embodiment, the present invention comprises a medical guidewire having an elongate body with distal and proximal sections. The body is constructed at least partially from a non-linear, superelastic nickel titanium alloy material having a nickel content in the range of about 50% to 51.5% atomic weight. The distal section of the body receives heat treatments in the range of 300xc2x0 C.-600xc2x0 C. for about 10 seconds to 60 minutes such that the material has recoverable strains in the range of about 1% to about 8%. The device also is provided with an occlusion device mounted on its distal section, and a lumen formed in the elongate body for communicating fluids from the proximal section to the distal section of the body. A passage way is formed through the distal section to communicate said fluids to the occlusion device.
In another embodiment, the present invention comprises a medical catheter having an elongate body and having distal and proximal sections. The body is constructed from a nickel titanium alloy material having a nickel content in the range of 50.0%-51.5% by atomic weight. At least the distal section of the body is constructed from a transformational nickel titanium alloy material exhibiting substantially constant stress over a range of recoverable strain from about 1% to about 8%. The device is also provided with a balloon mounted on its distal section and a lumen formed in the elongate body for communicating fluids from the proximal section to the distal section of the body. A passageway is formed through the distal section to communicate fluids to the balloon.
In yet another embodiment, present invention comprises a medical wire having an elongate body with distal and proximal sections. At least the distal section is partially constructed from a nickel titanium alloy material having substantially constant stress values over a range of recoverable strains from about 1% to about 8%. The proximal section is constructed from a second material having a modulus which is different for a given strain than the alloy material.
In yet another embodiment, the present invention comprises a medical wire having an elongate body comprising a hollow non-linear superelastic nickel titanium alloy material having recoverable strain in the range of 1% to at least 8%.
These and other advantages of the present invention will become more fully apparent from the following description taken in conjunction with the accompanying drawings.